Actuator-integrated force sensor

ABSTRACT

The present invention is based on an use of the already existing actuator bottom as a deformation element for a direct measurement of braking force, and on its geometric configuration in order to measure a force in a way which is largely independent of temperature and free of hysteresis. Accordingly, a force sensor is integrated into an actuator for generally or transmitting a force in the force flux. The actuator bottom is transverse to the force flux.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national application for International Application No.PCT/DE00/4266 which was filed on Nov. 30, 2000 and which published inGerman on Jun. 7, 2001 which in turn claims priority from 199 58 251.3,which was filed on Dec. 3, 1999 of which the following is a of which thefollowing is a specification.

FIELD OF THE INVENTION

The invention relates to a force sensor which is integrated into anactuator for generating or transmitting a force in the force flux andhas an actuator bottom that is transverse to the force flux.

BACKGROUND OF THE INVENTION

In motor vehicles, the braking function is implemented nowadays by meansof hydraulically activated actuators. In the method designated as“brake-by-wire”, electrically operated braking devices are used. In saidbraking devices, actuators, (i.e. elements in which the braking force isgenerated and by means of which the braking force is transmitted), areactivated by means of electromotive step-down drives. The resultingadvantages are the individual and variable configuration of the brakingprocess and the possibility of simultaneously performing furtherfunctions, for example the ABS function (Anti-lock Braking System). Asan electromechanical braking system will generally operate withcontrolled braking force, the precise measurement of the braking forceis essential to the performance of the overall system. High demands aremade on the precision of the system due to the synchronous operation.For example, fault tolerances should be <1% even if the braking forceis, for example, 5 t. The measuring task is additionally madeconsiderably more difficult as a result of limited accessibility torelevant measurement locations, the small amount of free space in thedirection of force and the extremely high spatial, and chronologicaltemperature gradients. These aspects rule out the use of known forcesensors such as, for example, strain gauges.

SUMMARY OF THE INVENTION

The present invention is based on the object of enabling precise andunambiguous sensing of braking forces as close as possible to thelocation where the braking force acts. More specifically, the inventionis based on the recognition that a force sensor can easily be integratedinto an actuator. The deflection of an actuator bottom or of a brakingpiston bottom (a designation by analogy with hydraulic systems) can beused as a measurement variable for the braking force. The actuatorbottom is appropriately configured for this purpose. The actuator isgenerally constructed in the form of a hollow cylinder, having anactuator bottom, and also containing a supporting ring by which it bearsdirectly or indirectly on the brake lining of a brake. The braking forceis generated centrally and applied to the actuator bottom.

The deformation of the actuator bottom is advantageously determined bymeans of various measuring methods. One method which is suitable forseries production is the capacitive measuring method, whereby theactuator bottom constitutes an electrode of a capacitor and thecapacitance which is changed with the deformation is determined. Thecapacitor will therefore generally be a plate capacitor. The electrodewhich lies opposite the actuator bottom is embodied as a plate and whichis pressed onto a base with spring support so that the high temperaturegradients do not cause any mechanical stresses to be transmitted to theinsulator of the electrode. Thus this ensures a defined electrodespacing, as is described in the European patent EP 0 849 576 B1.

The connecting point between the actuator bottom, namely the rear partof the actuator which is generally of cylindrical construction, and thesupporting ring, is embodied so as to be relatively rigid, since thebraking force can cause torques to be transmitted to the supporting ringat this point and said torques cause the measurement to be subject to ahysteresis due to friction effects. For this reason, the material crosssection at this connecting point is advantageously reduced by aninternal peripheral groove, an external peripheral groove, or by meansof a combination thereof so that only minimum torques are transmitted.

The measures which are provided for minimized hysteresis are likewisesuitable for suppressing in the axial direction a temperature gradientin the actuator bottom due to largely radial introduction of heat. Axialtemperature gradients can cause the actuator bottom to bulge in thedirection of the force to be measured and result in an incorrectmeasurement.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are described below withreference to drawings, in which

FIG. 1 illustrates an actuator bottom with supporting ring, isothermsand the heat flow being indicated;

FIG. 2 illustrates an actuator bottom in the state of rest and in thedeformed state;

FIG. 3 illustrates an actuator with grooves provided on the inner andouter peripheries in order to reduce the material cross section betweenthe actuator bottom and supporting rings;

FIG. 4 illustrates shows a view corresponding to FIG. 2, but with ahysteresis-free deformation path of the actuator bottom;

FIG. 5 illustrates an actuator with measuring elements for thedeformation ΔZ; and

FIG. 6 illustrates an actuator with capacitive measuring device for thedeformation ΔZ; and

FIG. 7 illustrates a schematic sectional view of a motor vehicle brakingsystem with a sensor integrated in the frictional engagement and in theactuator.

DETAILED DESCRIPTION OF THE INVENTION

A significant aspect of the invention consists in the integration of theforce sensor in the actuator, the actuator bottom being used as ameasuring element. The elastic deformation of the actuator bottom withcorresponding application of a force is thus the measurement variable atthis deformation element. The magnitude of the force can be inferredfrom the deformation.

In order to prevent temperature-induced deformation at the actuatorbottom in the measuring direction, i.e. in the direction of force, it isensured that at the connecting point between the actuator bottom andsupporting ring the application of the temperature or the introductionof the heat is such that temperature gradients are minimized in thedirection of force, which is equivalent to an approximately axialprofile of the isotherms formed in the direction of force. The heat flowwill thus run inwards almost exclusively in the radial direction.

In order to prevent hysteresis effects during various load changes inwhich the force is increased and decreased, the generation or torquesand their transmission to the supporting ring are minimized in atargeted fashion. This leads to the connecting point between theactuator and the supporting ring being embodied in a way similar to ajoint. As the actuator bottom serves as a diaphragm-like deformationelement, when force is applied to the connecting point between theactuator bottom and external cylinder or supporting ring, a torque willbe generated which has a center of rotation positioned within a Tconnection. This leads to a radial migration of the supporting face ofthe supporting ring on the brake lining. As a result of friction forcespresent at the supporting point, when the load is reduced the originalsupporting position is no longer reached, so that hysteresis effectsarise which prevent reproducible measurements. As a result ofappropriately formed grooves which are constructed on the periphery, thetransmission of torques at the point in question is prevented.

FIG. 1 shows a section through actuator 1, the actuator bottom 2 beingarranged perpendicularly with respect to the direction of force. Thedirection of force is illustrated in FIG. 2. In addition, FIG. 1 shows abase plane 12 as an application point for the force, a supporting ring3, a brake lining 18, and the direction of the heat flow. The actuator 1has an overall cylindrical shapes for the most part is a hollow cylindershape. The supporting ring 3 is arranged in the region of the outerperiphery of the actuator bottom 2, in the direction of force behind theactuator bottom 2. In order to guide the actuator bottom, thehollow-cylinder form is extended opposite the supporting ring 3, beyondthe actuator bottom 2 and counter to the direction of force.Furthermore, isotherms 11, which characterize various temperatures T₁ toT₄, are entered in the actuator bottom 2. The construction correspondingto FIG. 1 does not contain any sensor elements and does not have anyfeatures which can prevent temperature effects, or hysteresis effects.The central feature in FIG. 1 is that the heat flow Q, starting from thebrake system with the brake lining 18 on which the supporting ring 3rests, is introduced into the actuator bottom 2 in such a way thattemperature gradients occur in the direction of force in the actuatorbottom 2. This leads to temperature-induced deformations of the actuatorbottom, which results in incorrect measurement of the force.

FIG. 2 shows a view corresponding to FIG. 1 wherein the force F, and thebraking force, are shown schematically, as is the deformation of theactuator bottom 2 in the form of the deflected actuator bottom 2. Themaximum deflection ΔZ will occur in the center of the usually radiallysymmetrical component. The deformation which is shown will generate atorque at the connecting point between the actuator bottom 2 and thesupporting ring 3. The center of rotation 10 of said torque isdesignated. As a result of this torque, the surface of the supportingring 3 which rests on the brake lining 18 will be displaced outward whenforce is applied. The torques M are indicated schematically.

FIG. 3 s is a view corresponding to FIG. 1, wherein the heat flow isintroduced into the actuator bottom 2 virtually perpendicularly to thedirection of force, i.e. radially from the outside to the inside, bymeans of an inner peripheral groove 8 and an outer peripheral groove 9.This gives rise to isotherms 11 which are approximately parallel to theforce. As a result, no temperature-induced deformations occur.

FIG. 4 shows an arrangement corresponding to FIG. 2, in which measuresto eliminate torques M occurring when force is applied also take theform of peripheral grooves 8 and 9 between the actuator bottom 2 andsupporting ring 3. Here, the actuator bottom 2 can be deflected by amaximum absolute value of ΔZ without torques occurring at its outeredges which act on the supporting ring 3 and which cause its supportingface on the brake lining 18 to migrate outward. The material crosssection is correspondingly reduced by the grooves 8 and 9 so that ajoint-like construction is achieved.

FIG. 5 shows an actuator arrangement with a measurement of the actuatorbottom deflection ΔZ by different sensors. On the one hand, thedeflection of the actuator bottom 2 can be measured inductively oroptically with a contactless distance sensor 13. The contactless sensoris, for this purpose, mounted on the base plane 12 which is orientedperpendicularly with respect to the direction of force, and is thusdisplaced by ΔZ in accordance with the central region of the actuatorbottom 2. This displacement is carried out in a contactless way bymoving the sensor close to the actuator bottom 2.

A further measuring method includes the use of strain sensors 6 whichare suitable for higher temperatures. These sensors measure, as theirdesignation suggests, a strain ε, which occurs when a force F acts onthe actuator bottom 2. Metallic, semiconductor, or piezoresistive straingauges, as well as capacitive strain sensors with silicon surfacemicromechanics can be used as strain sensors. As before, the peripheralgrooves 8 and 9 are illustrated in FIG. 5, together with the bearing ofthe supporting ring 3 on the brake lining 18.

FIG. 6 shows the actuator 1 with a capacitive measuring arrangement,whereby ΔZ is again measured. The capacitive measuring arrangementcontains an electrode 5 which is positioned on an electrode mount 7. Theelectrode mount is pressed in its outer region onto a base 14 withspring support. The base 14 will remain fixed, even when force isapplied. The spring support is brought about by means of the spring 15which is supported on the rear cover 22. This ensures that the electrode5 is oriented approximately plane-parallel with respect to the actuatorbottom 2 in the position of rest. The actuator bottom 2 thus constitutesthe opposite electrode corresponding to the electrode 5. A change in thedistance between these two electrodes generates a signal which isproportional to ΔZ.

FIG. 7 shows the entire arrangement of a brake system which engages abrake disc 17. The brake linings 18 which are held together by the brakecaliper 16 are pressed on both sides against the brake disc 17 if aspindle 20 exerts a braking force on the actuator 1 by electromotivemeans via the motor 19. The electromotive drive is usually connected toa step-down gear mechanism. The spindle 20 transmits the braking forcecentrally onto the actuator bottom 2, the motor 19 being supported atthe rear on a part of the brake caliper 16. In addition, the capacitivesensor 24 is shown schematically. In the illustration corresponding toFIG. 7, it is possible to see the heat flow which is introduced startingfrom the contact faces between the brake disc 17 and brake lining 18rearward via the brake lining into the supporting ring 3 and via thelatter into the actuator bottom 2. As temperature differences of several100° C. can occur here, it becomes clear that temperature-induceddeformations can prevent reproducible measurements.

The following is to be noted with respect to the influence oftemperature and hysteresis. The influence of temperature on a brake canbe enormous as the actuator 1 is heated up considerably in severalseconds during the braking operation. The heat flow Q occurs hereexclusively via the supporting ring 3, and is then distributed into theactuator bottom 2. In the process, considerable axial temperaturegradients occur in the actuator bottom, which is illustrated in FIG. 1.This leads to temperature-dependent bulging ΔZ of the actuator bottom 2,and thus to an incorrect measurement. However, if a turning, in the formof a peripheral groove 8, is made in the interior of the supporting ring3, the heat flow is introduced virtually radially into the actuatorbottom 2, and a temperature-induced axial bulging ΔZ is thus precluded.

The hysteresis phenomena on the described actuator occur as a result ofa relatively rigid connection of the actuator bottom 2 to the supportingring 3. The centrally introduced braking force not only causes adeflection ΔZ at the actuator bottom 2 but also generates a torque Mcorresponding to FIG. 2. This torque ensures radial migration of thesupporting face of the supporting ring 3. However, when the loadingceases, a considerable hysteresis effect then occurs owing to theconsiderable frictional effects, and said hysteresis prevents, to acertain extent, the deformation ΔZ from being reversed in proportion tothe force F. According to the present inventions hysteresis is avoidedin that the rigid connection between the actuator bottom 2 andsupporting ring is considerably reduced in cross section. Furthermore,the connection between these two parts is arranged approximatelycentrally with respect to the supporting face, as illustrated in FIG. 4.Moreover, a material with a low hysteresis is used to manufacture thesensor. Special stainless special steels which can beprecipitation-hardened, for example of the type 17-4PH, are preferablyused. The measurement of the deformation ΔZ which is proportional to thebraking force is expediently carried out in relation to the edge of theactuator. For this purpose, inductive or optical methods can be used.Capacitive measuring principles, as illustrated in FIG. 6, are alsoparticularly suitable owing to the high temperatures. The correspondingchange in capacitance arises due to a braking-force-dependent change inthe electrode spacing with respect to the actuator bottom 2. Ameasurement signal is proportional to the deformation ΔZ, and thus tothe braking force F, results from the radial strain ε of the actuatorbottom 2. In this case, high-temperature measuring gauges,piezoresistive sensors, or capacitive micromechanical strain sensors aresuitable as strain sensors.

What is claimed is:
 1. A device for measuring force in a force flux,comprising a force sensor integrated in an actuator, said actuatorhaving a bottom which is formed transversely with respect to the forceflux and on which the force acts centrally, and a supporting ring fortransmitting the force, arranged peripherally on an outer edge of theactuator bottom in the direction of force, the actuator bottom capableof being deformed by the force and comprising part of the force sensor,whereby a detectable deformation of the actuator bottom constitutes ameasure of the force, the deformation comprising a central deflection ofthe actuator bottom relative to the edge of said actuator, and thedeflection being detectable by a plate capacitor configuration having anelectrode positioned on an electrode mount which is pressed, with springsupport, onto a base, and wherein the actuator bottom serves as acorresponding electrode.
 2. A device for measuring force in a forceflux, comprising a force sensor integrated in an actuator, said actuatorhaving a bottom which is formed transversely with respect to the forceflux and on which the force acts centrally, and a supporting ring fortransmitting the force, connected peripherally on an outer edge of theactuator bottom in the direction of force, and an internal peripheralgroove formed to reduce a cross section of the connection so thatminimized temperature gradients occur in the direction of force in theactuator bottom and so as to result in a reduced mechanical couplingbetween the actuator bottom and supporting ring so that transmission offorce from the actuator bottom to the supporting ring is minimized, theactuator bottom capable of being deformed by the force and comprisingpart of the force sensor, whereby a detectable deformation of theactuator bottom constitutes a measure of the force.
 3. The deviceaccording to claim 2, wherein the deformation is detectable by meansselected from one or more of the following: inductive or opticalmeasuring methods, metallic piezoresistive or semiconductor straingauges, and strain sensors with silicon surface micromechanics.